1. Field of the Invention
The present invention relates to an optical recording/reproducing technique for recording and reproducing information with the use of light, and more particularly to an apparatus and a method of recording and reproduction with the use of holography, and a spatial light modulator therefor.
2. Description of the Related Art
Conventionally, an optical recording/reproducing apparatus that records information with the use of light is utilized for recording and reproducing a large volume of data such as a high-density image data. Such optical recording/reproducing apparatuses which are already put into practical use are, for example, a magneto-optical recording/reproducing apparatus for a medium such as a Magneto Optical Disk (MO), and a phase-change optical recording/reproducing apparatus for a medium such as a Compact Disk Rewritable (CD-RW), a Digital Versatile Disk Random Access Memory (DVD-RAM).
In recent years, among the optical recording/reproducing apparatuses, a hologram recording/reproducing apparatus draws a particular attention as an apparatus capable of providing a further improvement in information recording density. In the hologram recording/reproducing apparatus, in general, information to be recorded is provided to an information beam as a two-dimensional pattern, and the information beam is interfered with a reference beam in an optical recording medium for recording of the information as an interference pattern (hologram). For reproduction, the reference beam alone is applied onto the recorded interference pattern in the same arrangement as at the recording, to retrieve the information as a diffraction image from the hologram. Thus, in the hologram recording/reproducing apparatus, information is recorded and/or reproduced as a two-dimensional pattern, whereby high-speed recording and reproduction of large-volume information can be achieved.
In view of further improvement in information recording density, various types of hologram recording/reproducing apparatuses are proposed. One example is a hologram recording/reproducing apparatus of volume hologram type. The volume hologram type hologram recording/reproducing apparatus is provided with an optical recording medium whose thickness is sufficiently larger than a wavelength of light, in order to allow recording of various interference patterns in a thickness direction as well as in a plane direction of the optical recording medium. Hence, the interference pattern can be three-dimensionally recorded in the optical recording medium. In other words, information can be recorded in the same region of the optical recording medium in a multiplexing manner, to increase the storage capacity.
Another proposed hologram recording/reproducing apparatus is a shift multiplexing type. In the shift multiplexing type hologram recording/reproducing apparatus, position of irradiation of the reference beam at the information reproduction is slightly shifted from those at the recording. Then, even when the recorded interference pattern is irradiated with the reference beam, due to the lack of phase matching between the reference beam and the interference pattern, the diffraction pattern cannot be obtained. When the reference beam is maintained in the position where the diffraction pattern is not obtained and further recording of interference pattern with another information beam is performed, a plurality of two-dimensional information can be recorded in a multiplex manner in the same recording area of the optical recording medium depending on the arrangement of the reference beam.
Still another proposed hologram recording/reproducing apparatus uses a spatially modulated reference beam. In a simple hologram recording/reproducing apparatus, a reference beam with an in-phase plane wave is used. In this hologram recording/reproducing apparatus, however, a spatially modulated reference beam is used. The recorded interference pattern is complicated and the phase matching condition for the reference beam and the interference pattern is strict. Hence, the higher recording multiplicity is achievable (Japanese Patent Laid-Open Publication No. 2002-123949, for example, discloses a hologram recording/reproducing apparatus employing a holography and using a recording reference beam whose phase is spatially modulated.). In a recently-proposed hologram recording technique, one spatial light modulator generates both the information beam and the modulated reference beam for the hologram recording (see, for example, Hideyoshi Horimai and Jun Li, “A novel Collinear optical Setup for Holographic data Storage System,” Technical Digest of Optical Data Storage Topical Meeting 2004, pp. 258-260).
In the conventional hologram recording/reproducing apparatuses as described above, however, the arrangement of the optical elements inside the apparatus, and a subtle fluctuation in the position of the optical recording medium with respect to the apparatus, for example, significantly affect the recording/reproduction, and the portability of the optical recording medium, and the compatibility among the apparatuses or the like are difficult to enhance. At insertion and removal of the optical recording medium to and from the hologram recording/reproducing apparatus, for example, sometimes a minute misalignment between the optical recording medium and the hologram recording/reproducing apparatus occurs. Such misalignment may cause reproduction error. Specifically in the spatial modulating type hologram recording/reproducing apparatus, the strict phase matching condition between the reference beam and the interference pattern makes the problem of the misalignment of the optical elements even more notable.
The conventional hologram recording/reproducing apparatus in general includes an optical recording medium, a recording/reproducing optical system that irradiates the optical recording medium with an information beam and/or a reference beam, and a spatial light modulator that generates the information beam and the reference beam. Hence, in order to improve the reproducibility at the irradiation of the optical recording medium with the reference beam, the misalignment between the optical recording medium and the spatial light modulator must be corrected. In the conventional technique, however, only a unit for correcting the misalignment between the optical recording medium and the recording/reproducing optical system is proposed and the correction of misalignment between the optical recording medium and the spatial light modulator is not possible.